Nickel-metal hydride storage batteries which use a negative electrode including a hydrogen storage alloy as a negative electrode active material has excellent output characteristics and high durability. Therefore, nickel-metal hydride storage batteries are expected to be used as a driving power source for electric vehicles, for example. Moreover, nickel-metal hydride storage batteries are capable of repeated use; and from increased awareness of environmental issues in recent years, they are attracting attention due to being capable of reducing environmental burden if used as an alternative to dry batteries.
For a nickel-metal hydride storage battery, an electrode assembly including a positive electrode, a negative electrode, and a separator interposed therebetween is housed in a metallic battery case (outer packaging can), together with an alkaline electrolyte. For a positive electrode active material, a nickel oxide such as nickel oxyhydroxide or nickel hydroxide is mainly used. The hydrogen storage alloy serving as the negative electrode active material is capable of reversibly absorbing and releasing hydrogen.
For the hydrogen storage alloy, that having a CaCu5-type crystal structure is mainly used. When high capacity is required of the hydrogen storage alloy, that including a Ce2Ni7-type or CeNi3-type crystal structure is used. In order to improve the battery characteristics of a nickel-metal hydride storage battery, attempts are being made to optimize the performance of the hydrogen storage alloy in powder form.
For example, Patent Literature 1 discloses use of a hydrogen storage alloy including a rare earth element, Mg, and Ni of which the equilibrium hydrogen pressure is increased, thereby to allow the operating voltage of a nickel-metal hydride storage battery using such alloy to increase, so that such battery can operate in a device designed for a battery operating voltage of 1.5 V to correspond with a dry battery.
In a nickel-metal hydride storage battery, if the negative electrode capacity is smaller than the positive electrode capacity, large amounts of hydrogen would be generated at the negative electrode during overcharge. Therefore, the negative electrode capacity is usually made larger than the positive electrode capacity, and by doing so, generation of hydrogen gas at the negative electrode during overcharge can be reduced. When the positive electrode capacity is smaller than the negative electrode capacity, the positive electrode active material that should react with hydroxide ions becomes completely consumed during overcharge; and the resultant oxidation of the hydroxide ions causes oxygen gas to be generated at the positive electrode.
However, since the negative electrode capacity is large (i.e., metal hydride that is active in a charge reaction is included in the hydrogen storage alloy, in large amounts), the negative electrode can absorb oxygen gas generated at the positive electrode and convert it to water. Specifically, at the negative electrode, the oxygen gas reacts with the metal hydride included in the hydrogen storage alloy and is converted to water. As such, in a nickel-metal hydride storage battery, the negative electrode capacity is made larger than the positive electrode capacity for the negative electrode to suppress generation of hydrogen gas during overcharge and to absorb oxygen gas, thereby to suppress rise in the internal pressure of the battery and allow hermeticity of the battery.
On the other hand, since the positive electrode capacity is smaller than the negative electrode capacity, at the positive electrode, during overdischarge, nickel oxyhydroxide that should react with water becomes completely consumed and the resultant reduction of water causes generation of hydrogen gas. The hydrogen gas generated at the positive electrode reaches the negative electrode by diffusion, is then absorbed into the hydrogen storage alloy. Specifically, at the negative electrode, the hydrogen gas is oxidized due to a reaction with the hydrogen storage alloy and is converted to water. As such, in a nickel-metal hydride storage battery, hydrogen gas generated at the positive electrode is absorbed into the negative electrode, thereby to suppress rise in the internal pressure of the battery during overdischarge.
Patent Literature 2 discloses providing a water-repellent resin on the surface of a negative electrode formed of a hydrogen storage alloy, as a technique to improve oxygen gas absorption and hydrogen gas absorption. Patent Literature 3 also proposes applying a dispersion including a water repellent agent to the surface of a separator using rollers, in order to prevent separation of the water repellent agent from the negative electrode surface.